The devices and techniques described herein were made in the performance of work under a NASA contract, and are subject to the provisions of Public Law 96-517 (35 U.S.C. xc2xa7202) in which the Contractor has elected to retain title.
This application relates to semiconductor radiation detectors, and in particular, to multi-quantum-well radiation detectors.
Semiconductor radiation detectors use optical absorption at optical transitions between two different energy levels to detect radiation by measuring the responses of the detectors caused by the optical absorption. The natural energy levels in extrinsically-doped semiconductors, such as doped silicon, may be used for detecting radiation. Properties of dopants and the host semiconductors may be selected to achieve desired detector performance, such as the detection spectral range, the spectral bandwidth, the responsivity, and the response time. However, the freedom in tailoring the detector performance is limited to the given natural properties of the dopants and the host semiconductors.
In another implementation, artificial multi-quantum-well structures are used to construct detectors. The structures and properties of the multiple quantum wells can be selected to achieve desired detector performance with greater flexibility and freedom than the extrinsically-doped semiconductor detectors. For example, an infrared quantum-well semiconductor detector usually includes a quantum-well structure formed of alternating active quantum well layers and barrier semiconductor layers. Such a quantum-well structure can have different energy bands each with multiple quantum states. An intraband transition between a ground state and an excited state in the same band (i.e., a conduction band or a valance band) can be used to detect infrared (xe2x80x9cIRxe2x80x9d) radiation by absorbing IR radiation at or near a selected resonance IR wavelength. The absorption of the radiation generates electric charge indicative of the amount of received radiation. The radiation-induced charge can then be converted into an electrical signal (e.g., a voltage or current) to be processed by signal processing circuitry.
The compositions of lattice-matched semiconductor materials of the quantum well layers can be adjusted to cover a wide range of wavelengths for infrared detection and sensing. Quantum-well structures can achieve, among other advantages, high uniformity, a low noise-equivalent temperature difference, large format arrays, high radiation hardness, and low cost. Infrared quantum-well sensing arrays may be used for various applications, including night vision, navigation, flight control, and environmental monitoring.
This application includes a multi-quantum-well (MQW) detector structure in a blocked intersubband detector (BID) configuration. The MQW structure is designed to operate on an intersubband transition for IR direction. Different from many MQW detectors where the MQW structure is sandwiched between an emitter contact and a collector contact layers, the BID device includes a thick blocking barrier layer between the MQW structure and the collector contact layer to block the dark current. In one embodiment, the thick blocking barrier layer is formed of an impurity-free semiconductor material.
When operating at low temperatures, e.g., about 20 to 30 K, the thermionic emission and thermally-assisted tunneling through the barrier layers in the MQW structure are suppressed. Hence, the carriers in the quantum wells are depleted by optical absorption. This carrier depletion causes the device to be less responsive and even becomes inoperative. To replenish the carriers to the MQW structure, the MQW structure is designed to have thin barrier layers to form a supperlattice MQW structure that support a ground state and an excited state minibands due to overlap of wavefunctions of adjacent quantum wells. This allows for sequential resonant tunneling of the electrons from the emitter contact layer. This tunneling refills the quantum wells and sustains the optical absorption of the MQW structure. The MQW structure is designed to have inter-subband transition from a bound state to a quasibound state within either a conduction band and valance band.
In one embodiment, the BID device includes an emitter contact layer, a multi-quantum-well structure, a blocking barrier layer, and a collector contact layer in contact with the blocking barrier layer. The multi-quantum-well structure has a first side in contact with the emitter contact layer and an opposing second side that is in contact with the blocking barrier layer. The multi-quantum-well structure is formed of alternating quantum well layers and barrier layers, where each barrier layer is of a thickness that allows for a spatial overlap of wavefunctions of adjacent quantum wells to permit a tunneling therethrough from one quantum well to an adjacent quantum well. In particular, a tunneling structure is implemented in the MQW structure to allow for sufficient electronic tunneling through the barriers of the MQW structure to replenish the absorption-depleted quantum wells. This carrier replenishment mechanism built into the MQW structure ensures that, the MQW structure, whose dark current is suppressed by the blocking barrier layer and the cryogenic condition, maintains sufficient carrier population (no space charge layers or no dielectric relaxation) in the quantum wells to be optically responsive to incident radiation. The blocking barrier layer has a thickness that substantially prohibits a tunneling therethrough.
A method according to one embodiment includes providing a multi-quantum-well structure to allow for carrier tunneling through a barrier layer from one quantum well to an adjacent quantum well to provide carriers for optical absorption, and preventing any carrier tunneling from the multi-quantum-well structure to a contact layer that receives carriers from the multi-quantum-well structure to reduce a dark current.